Function call needed instead of reference.

(and fix viewport scaling for non square viewers)

This also factorize the loading codepaths with and without slicing

Just checking boolean value would lead to a len() call, which might fail
as line_idxs might not have been set yet

self.layers dictionnary could change from another thread, thus changing
size during repaint iteration. Using a fixed range with the current
number of layers avoids iterating directly on the dictionnary.